In biogas plants, a combustible gas is produced by digesting organic matter in a digester, typically by methane formation. Such digesters are typically operated under an internal gas pressure which is generated by sealing the digester gas-tightly and by controlled extraction of the biogas produced therein. Digesters may be loaded continuously or semi-continuously (batch-wise) with organic solids in order to keep the digestion process going. Both during loading and also after loading, the pressure inside the digester must be overcome and maintained, and undesired leakage of materials from the digester must be prevented.
An apparatus for introducing livestock waste into a liquid manure digester is known from DE 202 16 090 U1, in which the livestock waste is filled into the digester from above by means of a plurality of screw conveyors. A similar apparatus for loading a digester vessel of a biogas plant is known from DE 299 03 208.
In addition to these prior art apparatuses for loading a digester vessel from above, devices for loading the digester vessel from below are also known which in many cases achieve better mixing of the supplied solid material with the solid material already in the vessel.
A basic problem when feeding solid materials into the digester is that a reverse flow of solids, liquids or gases from the digester through the feed device must be reliably prevented. This prevention of reverse flow must be achieved not only when the feed device is in operation but also when the feed device is not in operation, for example when solid materials are semi-continuously fed in successive feed operations performed at intervals.
A conveying device for introducing organic solids into a digester vessel, said device comprising a screw conveyor having a section with no flighting, is known from DE 102 52 527 B4. The aim of this apparatus is to compress the material being conveyed in the region of the flightless section in order to seal the conveying screw against the reverse flow of solids, liquids and gas. Due to the flightless section in the screw conveyor, the feed rate of the screw conveyor is reduced, however, and in the case of solid materials which are difficult to convey, it is possible that the internal resistance in the screw conveyor increases to such an extent that the feed rate falls to zero, thus resulting in blockage.
A more advanced feeding device for feeding solid material into a digester vessel and which likewise has a flightless section and which is also equipped with an axially movable flighting is known from DE 20 2010 000 550 U1. This axial mobility is aimed at preventing blockages of the screw conveyor. Although the device has proved to be advantageous in this respect, it requires complex and error-prone mounting and driving of the conveying screw in order to achieve the superimposed axial mobility and rotation.